1. Field of the Invention
The present invention relates to an antenna device, which has a radiator and a reflector, for combining a radio wave arriving from an incoming direction and a reflected wave from the reflector with each other with the radiator, and receiving the combined wave, an antenna reflector, and a wireless communication unit incorporating an antenna therein, and more particularly to an antenna device using a planar reflector for achieving high directivity, an antenna reflector, and a wireless communication unit incorporating an antenna therein.
More specifically, the present invention is concerned with an antenna device having a high front-to-rear ratio and a small size, an antenna reflector, and a wireless communication unit incorporating an antenna therein, and more particularly to an antenna device having a high front-to-rear ratio though a planar reflector has reduced dimensions, an antenna reflector, and a wireless communication unit incorporating an antenna therein.
2. Description of the Related Art
An array antenna, typified by a Yagi antenna (see, for example, Japanese patent Laid-open No. Hei 11-68452) has an element, called a feed element, connected to a power supply and another passive element not connected to a power supply for better antenna characteristics.
The phase of the current of the passive element varies depending on the distance between the elements and the length of the elements due to the combination with the feed element. For example, if an array antenna includes two elements, i.e., a pair of feed and passive elements, then the passive element may be spaced about a quarter of the used wavelength λ from the feed element in a direction opposite to the radio wave incoming direction. The feed element combines the radio wave from the incoming direction and a reflected wave from the passive element in phase with each other for receiving the radio wave with a high antenna gain.
The phase of the current of the passive element depends on the length of the passive element. If the length of the passive element is equal to or greater than one half λ/2 of the used wavelength λ, then the phase of the current of the passive element is of about 90 degrees. If the length of the passive element is smaller than one half λ/2 of the used wavelength λ, then the phase of the current of the passive element is of about 220 degrees.
If the length of the passive element is equal to or greater than λ/2, then the electric field is greater in a forward direction from the passive element to the feed element, exhibiting such a directivity that a radiated radio wave looks as if it is reflected by the passive element and radiated toward the feed element, so that the antenna has a high front-to-rear ratio. In this case, the passive element serves as a reflector, and the feed element as a radiator.
If the length of the passive element is smaller than λ/2, then the electric field directed in the forward direction from the passive element to the feed element is smaller, and the electric field directed in a rearward direction from the feed element to the passive element is greater. In this case, the passive element is considered to guide the radiation of the radio wave, and serves as a waveguide rather than a reflector. The antenna has a low front-to-rear ratio.
For constructing a reception antenna including a radiator and a reflector and having a high front-to-rear ratio, some size limitations are imposed on the reception antenna such that the distance between the radiator and the reflector be of a quarter of the used wavelength λ and the dimension of the reflector in the direction of the electric field be of λ/2 or greater.
Array antennas such as Yagi antennas are capable of developing a maximum electric field in the direction of the array between two antenna elements whose power supply polarities are inversely connected (reverse-phase array). Such an antenna arrangement is called “end-fire array antenna”.
Another example of end-fire array antenna is a dipole antenna with a reflector plate (see, for example, Japanese patent Laid-open No. Hei 6-268433). The dipole antenna includes a conductor plate acting as a reflector and a dipole antenna disposed parallel to the surface of the conductor plate. An image antenna is produced behind the conductor plate in reverse phase with the dipole antenna, making the array antenna equivalent to a reverse-phase dipole array antenna. A radio wave radiated from the dipole antenna is radiated forwardly of the conductor plate as if reflected by the conductor plate. The single antenna provides a directivity equivalent to that of a two-element array antenna.
A corner reflector antenna (see, for example, Japanese patent Laid-open No. Hei 9-153736) includes a reflector, i.e., a corner reflector, in the form of a conductor plate bent through a predetermined angle ψ along a ridge in a main direction of polarization, and a dipole antenna extending parallel to the ridge and spaced from its vertex by a quarter of the used wavelength λ. An image antenna with respect to the dipole antenna is produced behind each of the surfaces of the bent conductor plate. One of the surfaces of the bent conductor plate produces an image conductor plate of the other surface, and the image conductor plate produces an image antenna with respect to the image antenna. Therefore, a total of three image antennas are produced, making the corner reflector antenna equivalent to four array antennas. The dipole antenna and its image antenna are in reverse phase with each other, and the image antenna and its image antenna are in reverse phase with each other. Consequently, the corner reflector antenna is equivalent to two sets of reverse-phase array antennas. The combined directivity of the four array antennas is determined according to the principles of the product of directivities. No radio wave is reflected rearwardly of the conductor plate, and the directivity is confined only in a small range sandwiched by the bent second plane, resulting in a unidirectional antenna with sharp directivity.
Recently, the wireless data communication technologies are finding their use in a rapidly spreading application range. For example, the Bluetooth communications, which are known as standards for providing wireless connection interfaces applicable to various industries, provide a wireless communication technology for connecting mobile terminals. For example, wireless links based on the Bluetooth communications may be applied to connect a main telephone set and a cordless handset, a portable music player and a headset, or a stereo component and a speaker.
Applications of the Bluetooth communications include data communications from a wireless microphone as an acoustic data source to a receiver unit as a sink for receiving and transferring acoustic data to a recorder, e.g., a video recorder. The wireless microphone allows the video subject to move in a range that is not limited by the microphone cord.
The reception antenna of the receiver unit is required to have sharp directivity toward the wireless microphone. Therefore, an antenna device having a planar reflector, such as a corner reflector antenna having a high front-to-rear ratio, is considered to be desirable for use as the reception antenna of the receiver unit.
If a receiver unit incorporates an antenna device therein, then the antenna device needs to be small in size. Since a radiator is disposed in front of a reflector, it is desirable to provide a space for accommodating other circuit components behind the reflector. The antenna device is required to have a high front-to-rear ratio in order to avoid the generation of an interference wave against the circuit components.
However, the antenna device is subject to such limitations that the distance between the radiator and the reflector be of about a quarter of the used wavelength λ and the dimension of the reflector in the direction of the electric field (polarizing direction) be of λ/2 or more, as described above. For example, it is assumed that the antenna device employs a rectangular planar reflector and has an operation frequency band of 2.45 GHz. If the dimension of the reflector in the direction of the polarizing direction is smaller than 0.4λ, then the reflector does not operate, and the antenna device has a poor front-to-rear ratio. The dimensional lower limit is 0.4λ for a sheetmetal reflector, and 0.4λ/√{square root over (∈)}eff for a reflector with a dielectric substance having a dielectric constant ∈eff. If the front-to-rear ratio is poor, then operation of the circuit module disposed behind the reflector tends to be adversely affected.